This invention relates to film projectors, and is more particularly concerned with a device for inserting a film-protective filter into the light path of a projector during still projection of a cinematographic film into the light path of a projector during still projection of a cinematographic film or during prolonged projection of film slides or the frames of a film strip.
In the synchronized operation of a film projector of the type wherein film advance is controlled by pulse signals generated by a tape recorder, for example, if the time interval between one pulse signal and the next is prolonged, the particular film frame or slide being projected can become over-heated by the projection lamp, possibly causing permanent damage to the film. It has been proposed to insert a lightattenuating, heat-blocking filter into the light path between the projection lamp and the film to alleviate this condition, but is has not been possible heretofore to provide automatic insertion of such a filter when required to prevent film damage. Moreover, it has not been possible to control the automatic insertion and removal of such a filter in a manner which avoids unduly frequent variation of projected light levels.